Our long-term goal of this project is to elucidate the molecular mechanism of ATM in DNA damage and repair pathways as an attempt to further understand Ataxia Telangiectasia, an autosomal recessive genetic disease associated with loss-of-function mutations in the ATM gene. The ATM protein kinase is considered a critical element in determining cellular responses to ionizing radiation (IR). ATM is activated after DNA damage and signals many key regulators in DNA damage pathways by phosphorylation to initiate an optimal response. Of these, Protein Phosphatase 1 (PP1), a major eukaryotic protein serine/threonine phosphatase that regulates a variety of cellular function in response to DNA damage, is activated in an ATM-dependent manner after IR. However, detailed mechanisms on how ATM activates PP1 remain further explored. We have recently demonstrated that ATM is required for the rapid dissociation of PP1 from its regulatory subunit, Inhibitor-2 (I-2) in response to IR. Furthermore, we found that ATM phosphorylated I-2 at Serine 43 after DNA damage, leading to dissociation of the PP1/I-2 complex and activation of PP1. Our preliminary data in this proposal demonstrated that the ATM/PP1/I-2 pathway was required for the IR-induced S phase and G2/M checkpoints. Further, we found Brca1, Cdc7, and Chk2 presented in a complex with the phosphorylated form of I-2. To dissect the upstream and downstream elements of the ATM/PP1/I-2 pathway and establish links of the pathway to the cell cycle machinery, we propose three specific aims: 1). Investigate the role of Brca1 in ATM-mediated I-2 Ser 43 phosphorylation and PP activation in response to IR;2) Investigate the role of the ATM/PP1/I-2 pathway on Cdc7 inactivation and the S-phase checkpoint;3). Investigate the mechanism by which the ATM/PP1/I-2 pathway activates the G2/M checkpoint by studying I-2 mediated Chk2 activation and Chk2 phosphorylation/activation of PP1 in response to IR. Completion of these studies will provide insights into roles and mechanisms of the ATM kinase and, in the process, shed light on general mechanisms involved in the cellular response to DNA damage. PUBLIC HEALTH RELEVANCE: The goal of this proposal is to investigate the functional importance of an ATM- mediated signaling pathway in response to radiation-induced DNA damage as an attempt to further understand the cause of Ataxia-Telangiectasia (A-T), an autosomal recessive genetic disease associated with loss-of-function mutations in the ATM gene.